Investigations of the cellular organization and composition of the retina have provided an understanding of how humans visualize their surroundings and a framework necessary to cure human retinal diseases. Nevertheless, many questions about vertebrate retinal function remain unanswered. For example, although many of the components involved in cone photoreceptor light responses have been identified, the precise mechanisms by which these molecules function to define cone physiology as distinct from rod physiology are unclear. Also, although different cone types in the vertebrate retina are thought likely to express unique molecules, few such molecules have been identified. Identifying cell- type specific molecules would answer questions both about the formation of specific connections within the retina and questions about the establishment of cell-type position and identity. The goal of this proposal is use zebrafish mutants to answer these questions. We have developed a behavioral assay that efficiently identifies zebrafish mutants with subtle and specific defects in retinal function. 1). We will continue to isolate more mutations as a resource for understanding vertebrate retinal function. 2) We will characterize mutations in detail using biochemical, molecular and physiological approaches. We will focus initially on three mutations. Two of these, noa and nrb, produce abnormalities in cone photoreceptor physiology, and the third mutations, pob, causes the selective loss of red cone photoreceptors within the retina. Our characterization will help define the physiological responses of cone photoreceptors. 3.) We will also use the pob mutation to identify molecules specific to red-sensitive cone photoreceptors. 4). Finally, we will define the molecular nature of the defect in noa, nrb and pob by using a candidate gene or positional cloning approach. As a first step towards this end, we will generate a database of retina-specific genes and map these genes.